The texaphyrins are certain "expanded porphyrins" which have been found to be useful as MRI contrast agents, as radiation sensitizers and in photodynamic therapy (PDT), as well as having the ability to hydrolytically cleave phosphate esters, including RNA, and to photolyrically cleave RNA and DNA. Texaphyrin is considered as being an aromatic benzannulene containing both 18.pi.- and 22.pi.-electron delocalization pathways. See, e.g., Sessler, J. L. et al., Accounts of Chemical Research, 1994, 27, 43. Texaphyrins and water-soluble texaphyrins and method of preparation have been described in U.S. Pat. Nos. 4,935,498; 5,252,720; 5,256,399; 5,272,142; and 5,292,414; and in International publn.s WO 94/29316 and WO 95/01996; all of which are incorporated herein by reference.